Heat exchanger with improved extended surface



HEAT EXCHANGER WITH IMPROVED EXTENDED SURFACE Filed May 18, 1967 D. DALIN May 28, 1968 9 Sheets-Sheet 1 May 28, 1968 D. DALlN 3,335,355

% HEAT EXCHANGE WITH IMPROVED EXTENDED SURFACE Filed May 18, 1967' 9 Sheets-Sheet 2 3&5.

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May 28, 1968 D. DALIN 3,385,35fi

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May 28, 1968 D. DALIN $333,356

HEAT EXCHANGER WITH IMPROVED EXTENDED SURFACE 9 Sheets-Sheet 9 Filed May 18, 1967 Q" K f y 52 W 53 '52 W 5.3, 52 53 ,5? 55! 1 I I- w United States Patent Ofice 3,385,356 HEAT EXCHANGER WlTH IMPROVED EXTENDED SURFACE David Dalin, Vensberg, Sweden Continuation-impart of application Ser. No. 473,336, July 20, 1965. This application May 18, 1967, Ser. No. 639,544

Claims. (Cl. 165-146) ABSTRACT OF THE DISCLOSURE A heat exchanger for effecting indirect transfer of heat between two media separated by a metal wall, one of which media flows along a path partially defined by said wall, the wall having extended surface elements fixed thereon and projecting into the path of the fiowing medium, so that the temperature difference between the media diminishes along said path of flow. All of the extended surface elements have the same external surface area and for a substantial part of the length of said path the extended surface elements are bimetallic in cross section and consist of both high and low conductivity metal. The thermal conductivity of the elements is progressively less along said path in the direction of the downward gradient of the temperature difference, the reduction in thermal conductivity of the bimetallic elements being determined by the amount of high conductivity metal they contain.

This invention, like that of my copending application Ser. No. 473,336, filed July 20, 1965, now Patent No. 3,306,625, of which this application is a continuation-inpart, relates to heat exchangers for effecting indirect heat transfer between two media at opposite sides of a wall through which heat transfer can take place, and refers more particularly to heat exchangers of the extended surface type such as those illustrated in the Dalin Patents Nos. 2,469,635, 2,584,189, 2,655,352 and 2,719,354.

As fully explained in the Dalin et a1. Patent No. 2,469,- 635, the ability of a fluid medium to effect heat transfer between itself and a surface of a solid exposed to it, which ability is known as surface conductance, is not the same for all fluid media. Thus, for instance, the surface conductance of steam or water, i.e. boiler fluid, is at least one hundred times greater than that of combustion gases, and at 50 C. and with both media flowing at the same velocity, the surface conductance of oil is only nine percent (9%) that of water.

lt follows, therefore, that if maximum efliciency is to be obtained in a heat exchanger designed for indirect heat exchange between two media of different surface conductance, the partition wall separating the two media must have extended surface elements fixed to it and extending into the lower surface conductance medium, and preferably these extended surface elements are metal rods or wires, either round or substantially rectangular in cross section, but solidly welded at one end to the partition wall and projecting endwise therefrom.

The size, shape, distribution or arrangement of the extended surface elements and the thermal conductivity of the metal of which they are formed, all have a bearing upon the efficacy of the extended surface. The aforesaid Dalin et a1. Patent No. 2,469,635 went far in teaching the art how to use extended surface to increase the efliciency of heat exchanger. It taught the importance of and how to determine the most effective size, shape and distribution of the extended surface elements; and that the individual extended surface elements should be capable of conducting all of the heat abstracted thereby to the partition wall for transfer to the medium at the other side of the wall. To that end, and

3,385,356 Patented May 28, 1968 following the teachings of the Dalin et al. Patent No. 2,469,635, and the general knowledge of the art, it has been the practice to make all of the extended surface elements of metal having a high coefficient of thermal conductivity such as copper or aluminum, copper being by far the best choice.

For some purposes, though, as explained in the Dalin Patent No. 2,719,354, the copper was encased in a steel sheath or a steel core extended through the copper element.

In any event, all of the extended surface elements always had suflicient capacity to conduct all of the heat that could be abstracted thereby from the hottest medium flowing through the heat exchanger and across the elements. In other words, regardless of the amount of heat that was available to the elements, all of them had sufficient capacity to conduct an amount of heat that was available only in the hottest portion of the medium flowing across the elements. This meant that all of the extended surface elements contained the same amount of high conductivity metal although only a relatively small percentage of them needed it. Upon reflection, it can be readily appreciated that this constituted a very significant economic waste, since the cost of copper is high in comparison with that of suitable low conductivity metal such as steel.

It is, therefore, an object of this invention to eliminate this serious economic waste without sacrificing any of the efficiency of extended surface heat exchangers, by the simple expedient of using the maximum amount of high conductivity metal in only those of the extended surface elements that are subjected to the highest heat, and using proportionately less of the high conductivity metal in the elements that are located in the cooler zones of the heat exchanger.

Stated in another way, it is the object of this invention to provide a heat exchanger of the extended surface type in which the amount of high conductivity metal contained in the extended surface elements is substantially proportional to the temperature in the vicinity of the medium from which heat is being abstracted.

Thus in the heat exchanger of this invention the extended surface elements located in those portions of the system where the difference in the temperatures of the two media is greatest and where the maximum rate of heat transfer naturally occurs, have the greatest amount of high conductivity metal, while the elements in those portions of the system where minimum rates of heat transfer will occur, have the least amount of high conductivity metal and in fact may be formed entirely of iron or other suitable inexpensive low conductivity metal. This assures the lowest possible initial cost of the equipment without any sacrifice of operating efiiciency.

The attainment of lower cost is however not the only advantage derived from the use of less expensive ferrous metal for the extended surface elements in that part of the heat exchanger where the temperature differential between the two media is least.

Flue gases inevitably contain chemically reactive agents such as sulphur which upon condensation becomes sulphuric acid, and sulphuric acid, of course, has a serious corrosive effect upon metal. For some reason iron can withstand high concentrations of sulphuric acid much better than copper, whereas copper is much more resistant to low concentrations of sulphuric acid. Therefore, by using iron for the elements in that part of the system where temperature is lowest and maximum condensation takes place, as in the downstream end portion of the gas pass, and restricting the use of copper, either solid or steel clad, to the portion of the system where the temperature is greatest, a two fold advantage is gained-cost reduction and longer life for the extended surface.

With the above and other objects in view which will appear as the description proceeds, this invention resides in the novel construction, combination and arrangement of parts substantially as hereinafter described and more particularly defined by the appended claims, it being understood that such changes in the precise embodiment of the herein-disclosed invention may be made as come within the scope of the claims.

The accompanying drawings illustrate several complete examples of the physical embodiments of the invention, constructed according to the best modes so far devised for the practical application of the principles thereof, and in which:

FIGURE 1 is a vertical sectional view through a steam or hot water boiler equipped with extended surface in accordance with this invention;

FIGURE 2 is a cross sectional view through FIG- URE 1 on the plane of the line 22;

FIGURE 3 is a horizontal cross sectional view through a hot air furnace, also equipped with the extended surface of this invention;

FIGURE 4 is a vertical sectional view through the hot air furnace on the plane of the line 4-4 in FIG- URE 3;

FIGURE 5 is a vertical sectional view through FIG- URE 3 on the planes indicated by the lines 55;

FIGURE 6 is a horizontal sectional view through a water heater especially adapted to utilize waste heat, showing another form of extended surface embodying this invention;

FIGURE 7 is a longitudinal sectional view through FIGURE 6 on the plane of the line 77;

FIGURE 8 is a fragmentary sectional view through a portion of a heat exchanger equipped with one form of the extended surface elements of this invention;

FIGURE 9 is a sectional view through FIGURE 8 on the plane of the line 9-9;

FIGURES 10 and 11 are views similar to FIGURE 8 illustrating other forms of extended surface elements embodying this invention;

FIGURE 12 illustrates still another form of extended surface embodying this invention;

FIGURE 13 is a view illustrating how the extended surface of FIGURE 12 may be produced;

FIGURE 14 is a sectional view through FIGURE 12 on the plane of the line 14-14;

FIGURE 15 is a graph portraying the thermal conductivity of bimetallic extended surface elements having different proportions of high and low conductivity metal;

FIGURE 16 is a sectional view through several successive portions of a partition wall equipped with bimetallic extended surface elements of progressively lower thermal conductivity in accordance with this invention;

FIGURE 17 is a plan view of the structure shown in FIGURE 16;

FIGURES 18 and 19 are diagrammatic views illustrating one step in the production of the bimetallic elements shown in FIGURES 16 and 17; and

FIGURES 20, 21, 22 and 23 are cross sectional views through the extended surface elements of FIGURE 16, as indicated thereon by the lines 20-26; 21-21, 22-22 and 23-23.

Referring now more particularly to the accompanying drawings, and especially to FIGURES 1 and 2 thereof, the numeral 5 designates generally a steam boiler or hot water generator having a combustion chamber 6 in the lower portion of an inner shell 7, which as shown may be fired by an oil burner. An outer shell 8 surrounds the inner shell and cooperates therewith to provide a water space 9 provided with an inlet 10 and an outlet 11. As is customary, the outer shell is jacketed as at 12 to provide heat insulation and, as seen in FIGURE 2, the entire structure is cylindrical in cross section.

In the upper portion of the inner shell, above the combustion chamber, there is a water drum 13 connected at its top and bottom with the water space 9, as at 14 and 15. The annular space 16 between the drum 13 and the inner shell communicates the combustion chamber with a flue gas outlet 17 so that the hot combustion gases emanating from the combustion chamber sweep across the walls of the water drum 13 and the annular water space 5 in their passage to the outlet 17.

Inasmuch as the surface conductance, or in the terms of the aforesaid Dalin et al. Patent No. 2,469,635, the alpha value of water is about one hundred times greater than that of a gaseous medium, such as the hot flue gases flowing across the walls of the drum 13, the cylindrical wall. of the water drum has extended surface elements 18 fixed thereto and projecting therefrom substantially across the entire annular space 16. The size, shape and distribution of; these extended surface elements 18 is such that the ability of the elements to effect heat transfer between themselves and the combustion gases in contact therewith is substantially uniform throughout the entire length of the path of these gases as they flow upwardly across the cylindrical wall of the water drum. Of course, in this case, the heat exchange between the combustion gases or flowing fluid medium in contact with the extended surface elements is abstraction of heat from the gases and transfer thereof to the water in the drum 13.

From the standpoint of this invention, it is important to observe that as the hot combustion gases flow along the path defined by the space between the side walls of the inner shell 7 and the water drum i3, and the extended surface elements abstract heat energy therefrom and transfer it to the water, the difference in temperature of the two media diminishes.

In other words, there is a downward gradient in the temperature differential between the two media along the path of the flowing medium.

It is self evident that for the desired results to be attained, the individual extended surface elements must have the ability to conduct the heat which they abstract, to the wall of the drum 13 and through it to the water in the drum. Whether or not the elements possess this needed capability depends upon the thermal conductivity, i.e. the lambda value, of the metal of which they are made. The need for using metal having a sufficiently high lambda value so as to assure the needed conductivity has been understood for years and is fully discussed in the Dalin et al. Patent No. 2,469,635; but at the time the invention covered by that patent was made and developed, it was not appreciated that maximum thermal conductivity is needed only for those extended surface elements which are located in that part of the system at which the greatest temperature differential between the media exists and the maximum rate of heat transfer takes placeor, the corollary of this observation-that at the opposite end of the flow path, the thermal conductivity of the extended surface elements can be far less without in anywise affecting the efficiency of the heat exchanger.

Wherever extended surface has been used before, the individual elements have always been made either exclusively of high conductivity metal or, when laminated as in the Dalin Patent No. 2,719,354, the amount of high conductivity metal was the same in all of the elements.

Since only a relatively few of the extended surface elements of any heat exchanger equipped with extended surface need to cope with the maximum rate of heat transfer, it is evident that providing all of the elements with maximum conductivity involves a needless use of high conductivity metal. This in turn constitutes a serious economic waste because copper-which is most widely used where high conductivity is required-costs considerably more than suitable low conductivity metal, for instance, iron or steel.

In keeping with the objective of this invention as hereinbefore expressed, which is to reduce the cost of extended surface heat exchangers by eliminating unnecessary use of costly high conductivity metal in the extended surface elements, the elements 18 in the boiler 5 are divided into four groups designated 1, II, III and IV, as indicated in FIGURE 1. The only difference between these several groups of elements is in their thermal conductivity, and this difference may be obtained in a number of ways, some of which are shown in FIGURES 8 to 11, inclusive, in all of which the amount of high conductivity metal used is greatest in Group I and progressively less in the succeeding groups.

Thus the desired gradation in thermal conductivity in the succeeding groups may be obtained as shown in FIG- URE 8, wherein all of the extended surface elements are bimetallic and consist of a high conductivity core 19 encased in a sheath 20 of low conductivity metal, as in the Dalin Patent No. 2,719,354. Preferably the core is formed of copper, though it could be made of aluminum, while the sheath is formed of mild steel. In the bimetallic elements of Group I, the proportions of high to low conductivity metal may be 85 to 15. This gives the elements ample thermal conductivity to successfully cope with the highest temperatures attained by the combustion gases flowing over the extended surface.

For succeeding groups, the percentage of high conductivity metal to low conductivity metal present in the elements is reduced stepwise so that the thermal conductivity of the elements approximately matches the declining temperature gradient of the combustion gases as the heat thereof is abstracted therefrom. As long as the thermal conductivity, that is, the lambda value, of the elements is suflicient to conduct all of the heat abstracted thereby to the wall of the water drum, the performance requirements are met; but to gain the advantages of this invention the lambda value of the elements should not greatly exceed the performance requirements.

Reference to the chart of FIGURE 15 will show that the relative proportions of copper and iron in laminated elements should be such as those of FIGURE 8, to safely cope with the temperatures encountered and still gain the economic advantage of this invention. To illustrate, Example I on this chart depicts the situation for extended surface elements having a 5 mm. outside diameter, a copper core of 4.5 mm. diameter, and a 0.25 mm. thick iron sheath. In this case, the copper content is 82% and the iron content 18% of the whole; and as shown in the chart, the lambda value of the element is 283 K. caL/ m./h. C.

In Example II, the proportions of copper to iron are reversed from what they are in Example I. In this case, the lambda value is 94.

For other proportions of copper and iron, the lambda value can be readily determined from the chart.

Reversing the relationship between the copper and iron, that is, using the iron as the core material and copper for the sheath, does not affect the lambda value of the elements. Such reversal is illustrated in FIGURE 10, which, incidentally, includes a fifth group V. In this case, the elements of Group I, the highest conductivity group, are of solid copper and the elements of Group V are of solid iron, and hence have the lowest thermal conductivity, while the elements of the intervening groups II, III and IV have iron cores of stepwise increased diameter Within copper sheaths with correspondingly thinner walls.

The use of iron for the elements that are located in that part of the system Where the temperature differential between the two media is least and copper where the temperature differential is greatest, is especially advantageous in the case of the convection surfaces of steam boilers where the extended surface elements are swept by sulphur containing flue gases, for as noted hereinbefore, iron is better able than copper to withstand the higher concentrations of sulphuric acid resulting from the greater condensation that takes place in the downstream portion of the gas pass, whereas copper is more resistant to the lower concentrations of sulphuric acid in the higher temperature portion of the gas pass.

Another way of gaining the desired stepwise reduction in amount of high conductivity metal used for the extended surface elements is illustrated in FIGURE 11, wherein the elements of Group I are of solid copper and all the rest are tubular with stepwise reduced wall thicknesses. The use of such hollow elements is particularly advantageous when it is desired to have a large heat collecting or dissipating surface exposed to one medium and a minimum amount of material for conveying the heat to or from the other medium.

It should be understood that the structures shown in FIGURES 8 to 11, inclusive, are merely illustrative and that in actual practice there are many more extended surface elements arranged or distributed along the path of the medium flowing thereover. Also, in each of these views the numeral 22 designates the wall separating the two media between which heat exchange is to take place, which in the case of the boiler 5 in FIGURES l and 2, is the Wall of the water drum 13; and that the numeral 23 designates a part of the means which confines the flowing media to a defined path across which the extended surface elements project.

In the boiler of FIGURES 1 and 2, the two media between which heat exchange takes place have widely different surface conductance, so that extended surface projects from one side of the partition wall separating the two media. This would not be the case where the media at opposite sides of the partition wall were both gaseous as in the direct fired hot air heater shown in FIGURES 3, 4 and 5. In this unit, walls 25 separate a combustion chamber 26 and a combustion gas chamber 27 from an air chamber 28, and extended surface elements 29 and 39 are fixed to and project from opposite sides of the walls 25. By virtue of these extended surface elements, optimum heat transfer from the hot combustion gases to the air to be heated is obtained.

In keeping with the present invention, the extended surface elements on both sides of the partition walls are divided into the four groups I, II, III and IV, heretofore described, with the Group I elements located in the hottest portions of the zones to which the heating medium and the medium to be heated are confined.

Preferably the air chamber is divided into several successively communicating passages by vertical walls 31 and 32 and horizontal walls 33 through which the air to be heated flows from an inlet 34 to an outlet 35, the arrangement being such that the air which abstracts heat from the extended surface elements 29 and 3t) flows downward, and hence countercurrent to the rising combustion gases.

In the boiler of FIGURES l and 2, as Well as in the direct fired air heater of FIGURES 3, 4 and 5, the partition walls separating the two media also form part of the structure by which the flowing medium in contact with the extended surface is confined to a defined path. Hence the grouping of the elements along the path of the flowing medium resulted in elements of different thermal conductivity being mounted on the same partition wall. However, in a unit such as that shown in FIGURES 6 and 7, this is not the case. Here, the flowing medium-for instance, spent flue gasespass through a duct 35 defined by suitable walls, the flow being upward in FIGURE 6. In the duct 35 is a group or bank of serpentine tubes 36, the ends of each of which are connected with inlet and outlet headers 37 and 38 so that water, boiler fluid or the like can be circulated through the tubes to be heated by the flue gases flowing through the duct 35.

The serpentine tubes 36 have extended surface elements as fixed thereto as in FIGURES l and 2 of the Dalin patent No. 2,584.189; and since the fluid heating medium flowing through the duct successively engages the several legs of each tube, the extended surface elements on the legs of the tubes first contacted by the flowing medium have greater thermal conductivity than those last con tacted by the flowing medium. This condition is depicted by Groups IV and V in FIGURE 6. Obviously, of course, in this unit the walls of the serpentine tubes 36 separate the two media from one another, and the medium to be heated flows through the tubes.

While, as pointed out hereinbefore, aluminum can be used as the high conductivity metal in the bimetallic extended surface elements it should also be understood that the desired gradation in conductivity of a series of elements may be obtained by using both copper core elements and aluminum core elements. Thus, those elements which must have the highest conductivity may have copper cores while those requiring lower conductivity may have aluminum cores. Since aluminum is substan tially less costly than copper such combinations of copper and aluminum core elements has a significant cost advantage.

In the various structures thus far described, the extended surface elements have been rods or wires, but the invention is not limited to that form of extended surface. For instance, the extended surface might take the form of flat fingers 45, as shown in FIGURES 12, 13 and 14-. These fingers may be formed by shearing strip stock 46 into two comb-like sections, as shown in FIG- URE 13, and to give the fingers their desired bimetallic formation, the strip can be a flattened tube with a core 47 of copper and :a mild steel sheath 48. This form of extended surface is especially suitable for use on tubes, since it combines the more effective finger-like shape with ease of attachment to the tubes, since the combs can be spirally wrapped around the tubes with their continuous back edges in contact with the tubes to which they are, of course, secured in good heat transfer relation.

Still another way of providing extended surface elements with tailor-made thermal conductivity is illustrated in FIGURES 16 to 23, inclusive. In this case, the individual elements 50 are cut from lengths of bimetallic bands or ribbons produced in the manner diagrammatically illustrated in FIGURES l8 and 19. Thus a bimetallic wire 51 that is round in cross section and has a core 52 of copper within a steel sheath 53, is flattened by passage between a pair of rolls 54, into a band or ribbon 55.

The band or ribbon 55 has flat sides and rounded parallel edges. To provide bands or ribbons of the desired thickness, the band or ribbon 55 is run through more closely spaced rolls, one pair of which, designated 54, is shown in FIGURE 19, the band or ribbon 55 which issues from this pair of rolls being thinner than the band or ribbon 55.

Repeated passage of this band or ribbon between more closely spaced rolls to produce bands or ribbons of successively less thickness effects elongation of the band or ribbon, but does not appreciably increase its width, as can be seen from a comparison of the width dimensions W in FIGURES 23. As a corollary, the cross sectional perimeter of the different thickness bands or ribbons is substantially the same, so that elements cut from such different thickness bands or ribbons have substantially the same external surface area per unit of length. The only significant dimensional difference between elements cut from such different thickness bands or ribbons, assuming they are all of the same length, thus is in their thickness and necessarily in the amount or volume of copper they contain; and since it is this factor which determines the thermal conductivity of the elements, it follows that elements having different values of thermal conductivity can be obtained from the same bimetallic wire 51. This is the case in the extended surface illustrated in FIGURES l6 and 17, wherein the elements 50 are divided into four groups, I, II, III and IV, those of the first group having the greatest thickness as shown by FIGURE 20, and hence the highest thermal conductivity, those of the last group being the thinnest,

8 as shown by FIGURE 23, and having the lowest thermal conductivity.

From the foregoing description taken with the accompanying drawings, it should be apparent that this invention, though simple in its implementation of the underlying concept, has great value to the industry. As is so often the case, very meritorious inventions and developments fail of commercial adoption because of their cost. The economic factor is ever present. Hence, when a discovery is made which removes the economic barrier to full scale adoption of a more efficient and more compact structure such as the heat exchangers of this invention, the achievement is noteworthy, to say the least.

By this invention, the cost of extended surface heat exchangers possessing all of the attributes of the various Dalin patents hereinbefore referred to, is reduced by more than fifty percent, without in anywise detracting from the efficiency or performance of the units.

What is claimed as my invention is:

1. A heat exchanger having wall means separating a flowing medium from another medium and coacting with other wall means to define a path of flow for the flowing medium, and having extended surface elements fixed to said wall means and projecting into said path of the flowing medium, said heat exchanger being characterized in that:

(A) the extended surface elements are of such size and shape and so distributed along said path that as the flowing medium wipes across them the heat transfer which takes place between the extended surface elements and the flowing medium effects a reduction in the difference in temperature between the media, which difference diminishes progressively from one end of said path to the other;

(B) all of the extended surface elements are bimetallic in cross section and composed of both high and low thermal conductivity metal; and

(C) the relative proportions of said two metals are progressively different along said path, with the elements having the highest proportion of high thermal conductivity metal at the end of said path at which the temperature difference between the media is greatest,

so that the thermal conductivity of the extended surface elements substantially matches the downward gradient of the difference in temperature of said media along said path.

2. A heat exchanger having wall means separating a flowing medium from another medium and coacting with other wall means to define a path of flow for the flowing medium, and having extended surface elements fixed to said wall means and projecting into said path of the flowing medium, said heat exchanger being characterized in that:

(A) the extended surface elements are of such size and shape and so distributed along said path that as the flowing medium wipes across them the heat transfer which takes place between the extended surface elements and the flowing medium effects a reduction in the difference in temperature between the media, which difference diminishes progressively from one end of said path to the other;

(B) all of the extended surface elements except those at that end portion of said path at which the temperature difference between the media is least, are bimetallic in cross section and composed of high and low thermal conductivity metal;

(C) the relative proportions of said two metals are progressively different along said path, with the elements having the highest proportion of high thermal conductivity metal at the end of said path at which the temperature difference between the media is greatest,

so that the thermal conductivity of the bimetallic extended surface elements substantially matches the downward gradient of the difference in temperature of said media in contact therewith; and

(D) the extended surface elements at the end portion of said path at which the difference in temperature between the media is least are formed entirely of low thermal conductivity metal. I

3. A heat exchanger having wall means separating a flowing medium from another medium and coacting with other wall means to define a path of flow for the flowing medium, and having extended surface elements fixed to said wall means and projecting into said path of the flowing medium, said heat exchanger being characterized in that:

(A) the extended surface elements are of such size and shape and so distributed along said path that as the flowing medium wipes across them the heat transfer whch takes place between the extended surface elements and the flowing medium effects a reduction in the difference in temperature between the media, which difference diminishes progressively from one end of said path to the other;

(B) all of the extended surface elements except those located at the opposite end portions of said path are bimetallic in cross section and composed of both high and low thermal conductivity metal;

(C) the relative proportions of the two metals of said bimetallic elements are progressively different along said path with the elements having the highest proportion of high thermal conductivity metal near the end of said path at which the temperature difference between the media is greatest, and vice versa;

(D) the extended surface elements at that end portion of said path at which the temperature diflerence between the media is greatest are formed entirely of high thermal conductivity metal; and

(E) the extended surface elements that are located at the end portion of said path at which the temperature difference between the media is least are formed entirely of low thermal conductivity metal,

so that the thermal conductivity of the extended surface elements substantially matches the downward gradient of the difference in temperature of said media along said path.

4. The heat exchanger of claim 1, wherein the high thermal conductivity metal is copper and the low thermal conductivity metal is iron.

5. The heat exchanger of claim 2, wherein the high thermal conductivity metal is copper and the low thermal conductivity metal is iron.

6. The heat exchanger of claim 3, wherein the high thermal conductivity metal is copper and the low thermal conductivity metal is iron.

7. A heat exchanger having wall means separating a hot gaseous flowing medium from another medium to be heated, and defining a path of flow for the hot medium having upstream and downstream ends, the wall means having extended surface elements fixed thereto and projecting into the hot gaseous flowing medium to cooperate with the wall means in effecting indirect heat exchange between the media, said heat exchanger being characterized in that:

(A) the extended surface elements are divided into groups arranged in succession along said path, with each group comprising a series of banks of elements spaced along said path;

(B) all of the extended surface elements in each group have the same size, shape and thermal conductivity; but

(C) the extended surface elements comprising the successive groups have different values of thermal conductivity,

those of the group at the upstream end of said path where the temperature difference between the media is greatest being formed substantially entirely of high conductivity metal and having the maximum thermal conductivity, those of the group at the downstream end of said path where the temperature difference between the media is least being formed substantially entirely of low conductivity metal and having the minimum thermal conductivity, and those of the intervening groups are bimetallic in cross section and are composed of both high and low thermal conductivity metal,

the relative proportions of said two metals being progressively different along said path with the bimetallic elements having the highest proportion of high thermal conductivity metal adjacent to the extended surface elements formed substantially entirely of high thermal conductivity metal, and vice versa.

8. The heat exchanger of claim 7, wherein the extended surface elements that have the maximum thermal conductivity are formed of copper, the extended surface elements that have the minimum thermal conductivity are formed of ferrous metal, and the bimetallic extended surface elements are formed of copper and ferrous metal.

9. A heat exchanger having wall means separating a hot gaseous flowing medium from another medium to be heated, and defining a path of flow for the hot medium having upstream and downstream ends, the wall means having extended surface elements fixed thereto and projecting into the hot gaseous flowing medium to cooperate with the wall means in effecting indirect heat exchange between the media, said heat exchanger being characterized in that:

(A) all of the extended surface elements along the entire length of said path have the same external size and shape;

(B) the extended surface elements have different values of thermal conductivity along said path;

(C) the extended surface elements at the upstream end of said path where the temperature difference between the media is greatest are solid in cross section and have the maximum thermal conductivity; and

(D) the remaining extended surface elements are tubular in cross section and hence have less thermal conductivity.

10. The heat exchanger of claim 9, wherein the wall thickness of the tubular element is progressively less along the path of the flowing medium in the direction of the declining temperature difference between the media,

so that the thermal conductivity of the extended surface elements substantially matches the downward gradient of the difference in temperature of the media along said path.

11. A heat exchanger having wall means separating a flowing medium from another medium and coacting with other wall means to define a path of flow for the flowing medium, and having extended surface elements fixed to said wall means and projecting into said path of the longing medium, said heat exchanger being characterized in t at:

(A) the extended surface elements are of such size and shape and so distributed along said path that as the flowing medium Wipes across them the heat transfer which takes place between the extended surface elements and the flowing medium effects a reduction in the difference in temperature between the media, which difference diminishes progressively from one end of said path to the other;

(B) the extended surface elements along a substantial portion of the length of said path are bimetallic in cross section and composed of both high and low thermal conductivity metal;

(C) all of the bimetallic elements have substantially the same external surface area, but

(D) the thickness of the high conductivity metal portion of the bimetallic elements is progressively less along said path in the direction of the downward gradient of the temperature difference between the media, so that the thermal conductivity of the successive elements along said path substantially matches the downward gradient of the temperature difference.

12. The heat exchanger of claim 11 wherein the high thermal conductivity metal is copper and the low thermal conductivity metal is iron.

13. A heat exchanger having wall means separating a flowing medium from another medium and coating with other wall means to define a path of flow for the flowing medium, and having extended surface elements fixed to said wall means and projecting into said path of the flowing medium, said heat exchanger being characterized in that:

(A) the extended surface elements are of such size and shape and so distributed along said path that as the flowing medium wipes across them the heat transfer which takes place between the extended surface elements and the flowing medium effects the reduction in the difference in temperature between the media, which difference diminishes progressively from one end of said path to the other;

(B) the extended surface elements along a substantial portion of the length of said path are bimetallic in cross section and composed of both high and low thermal conductivity metal;

(C) all of said bimetallic elements are formed of band stock having flat sides and parallel edges, the width of which varies but little in the successive elements along said path but the thickness of which is progressively less along said path in the direction of the downward gradient of the temperature difference between the media,

so that while all of said elements have substantially the same cross sectional perimeter and hence substantially the same external surface area per unit of length, the cross sectional area of the elements and the thickness of the high conductivity portion of the elements and hence the thermal conductivity of said bimetallic elements is progressively less along said path in the direction of the downward gradient of the difference in temperature of said media.

14. The heat exchanger of claim 13, wherein all of said extended surface elements are bimetallic and formed of band stock with flat sides and parallel edges.

15. The heat exchanger of claim 14, wherein the high thermal conductivity metal is copper and the low thermal conductivity metal is iron; and

wherein the copper portion of the elements constitutes the core thereof.

References Cited UNITED STATES PATENTS 728,724 5/1903 Jones 165-183 761,927 6/1904 Norman 165183 2,055,549 9/1936 Modine 165-146 2,613,065 10/1952 Didier 16S146 2,719,354 10/1955 Dalin 165134 X 2,875,986 3/1959 Holm 165-166 X ROBERT A. OLEARY, Primary Examiner.

A. W. DAVIS, Assistant Examiner. 

